The growth and differentiation of the conceptus (embryo/fetus and associated extraembryonic placental membranes) is required in mammals for successful pregnancy. The placenta is essential for transfer of nutrients and gases to the embryo/fetus from the mother. Our long-term goal is to understand the cellular and molecular mechanisms of conceptus development and placental morphogenesis in order to provide fundamental knowledge that will be used to develop rational therapies for the prevention and clinical treatment of pregnancy loss and diseases involving placental dysgenesis, dysplasia, and dysfunction. This research proposal specifically focuses on the biological roles of endogenous retroviruses (ERVs) in conceptus development and placental morphogenesis. ERVs account for about 8% of the genome of every animal species. A number of ERVs are expressed in the human placenta and are associated with placental differentiation and pregnancy diseases such as preeclampsia. However, studies to determine the role(s) of ERVs in humans are not feasible for obvious ethical reasons. In this research project, experiments are designed to determine the biological role(s) of endogenous betaretoviruses during the peri-implantation period of conceptus development and placental morphogenesis in sheep. Sheep harbor endogenous betaretroviruses (termed endogenous Jaagsiekte Sheep Retroviruses or enJSRVs) that are specifically and highly expressed in the uterine endometrial epithelia and placenta. Hyaluronidase 2 (HYAL2), a cellular receptor for JSRV and enJSRV Env, is expressed only by the trophoblast giant binucleate cells and multinucleated syncytia of the placenta. Inhibition of enJSRVs Env expression in the sheep conceptus in vivo retards trophoblast growth, inhibits trophoblast giant binucleate cell differentiation, and compromises early pregnancy. The central hypothesis is that enJSRVs Env and HYAL2 regulate mononuclear trophoblast cell proliferation, differentiation of binucleate cells, and formation of multinucleated syncytia. The experiments utilize novel ideas and research approaches to determine the biological role of enJSRVs and the Hyal2 cellular receptor in trophoblast growth and differentiation. Successful completion of the experiments is expected to elucidate regulatory mechanisms controlling fundamental processes essential for the establishment and maintenance of pregnancy that are regulated by endogenous retroviruses.